Method for depositing protective coatings on carbon articles

ABSTRACT

The invention relates to a method and apparatus for coating carbon articles, such as carbon electrodes for use in steelmaking, with aluminium and refractory materials. The method comprises in a single operation, spraying aluminium in a molten state on to the surface, and immediately spraying refractory material on to the aluminium coating so formed. The apparatus includes spray heads, by which the aluminium and refractory materials are sprayed, the spray heads being located alongside each other and moved relatively to the carbon electrode simultaneously, the distance apart of the spray heads being sufficient to ensure that there is virtually no interference between the sprays of aluminium and refractory material, and also to ensure that the heat of the molten aluminium does not dry out the refractory material before it reaches the surface of the carbon article.

United States Patent 1 Hinchcliffe et al.

1 1 METHOD FOR DEPOSITING PROTECTIVE COATINGS ON CARBON ARTICLES [75] Inventors: Alan IIinchcliffe, Sheffield; Kenneth Johnson, Doncaster, both of England [73] Assignee: British Steel Corporation, London,

England [22] Filed: Oct. 20, 1972 [21] Appl. No.: 299,455

[30] Foreign Application Priority Data Oct. 22, 1971 Great Britain 49261/71 [52] US. Cl. ..117/217,117/93.l,117/104 R, 117/10413, 117/105, l17/l05.2, 117/l05.4,

[51] Int. Cl B44d l/08, B44d l/l6, C230 7/00 [58] Field of Search 117/228, 105, 217, 105.2, 1l7/93.l PF,104,105.3,105.4; 13/18 [56] References Cited UNITED STATES PATENTS 1,256,599 2/1918 Schoop 1l7/l05.4 1,978,415 10/1934 1 Collins 3,019,014 l/1962 Miksis ..1l7/l05.4 3,348,929 10/1967 Valtschev et al. 117/228 3,476,586 11/1969 Valtschev et al. 117/228 3,553,010 1/1971 Rubisch 117/217 FOREIGN PATENTS OR APPLICATIONS 1,026,055 4/1966 Great Britain 1,151,071 5/1969 Great Britain 1451 Feb.4, 1975 Primary Examiner-Cameron K. Weiffenbach Attorney, Agent, or Firm-Cushman, Darby & Cushman [57] ABSTRACT The invention relates to a method and apparatus for coating carbon articles, such as carbon electrodes for use in steelmaking, with aluminium and refractory materials.

The method comprises in a single operation, spraying aluminium in a molten state on to the surface, and immediately spraying refractory material on to the aluminium coating so formed.

The apparatus includes spray heads, by which the aluminium and refractory materials are sprayed, the spray heads being located alongside each other and moved relatively to the carbon electrode simultaneously, the distance apart of the spray heads being sufficient to ensure that there is virtually no interference between the sprays of aluminium and refractory material, and also to ensure that the heat of the molten aluminium does not dry out the refractory material before it reaches the surface of the carbon article.

9 Claims, 2 Drawing Figures PATENTEDFEB sum 10F 2 FIG; 7.

METHOD FOR DEPOSITING PROTECTIVE COATINGS ON CARBON ARTICLES BACKGROUND OF THE INVENTION This invention relates to methods of and apparatus for creating protective coatings on the surfaces of carbon articles. The invention is a modification of the basic process described and claimed in British Pat. Nos. 1,026,055 and 1,15 1,071. In these patent specifications are described methods of coating carbon articles with aluminum layers and then applying to the aluminium layer a layer of refractory material. As practised up to date these methods have involved applying the aluminium by use of an electric arc to atomise the aluminium and compress air to blow the atomised aluminium on to the surface of the carbon article. The article is then allowed to cool sufficiently to paint on to the aluminium surface a slurry containing refractory material.

SUMMARY OF THE INVENTION We have now found that it is not only possible to spray the refractory material on to the aluminium coating but that we can carry out the two coating steps in a single operation with consequent saving in time and effort.

According to the invention a method of creating a protective coating on the surface of a carbon article comprises, in a single operation, spraying aluminium in molten state on to the surface and immediately spraying refractory material on to the aluminium coating so formed.

The spray heads by which the aluminium and refractory materials are sprayed can be located alongside each other and moved relatively to the carbon electrode simultaneously, the distance apart of the spray heads being sufficient to ensure that there is virtually no interference between the sprays of aluminum and refractory material, and also to ensure that the heat of the molten aluminium does not dry out the refractory material before it reaches the surface of the carbon article.

Preferably, the refractory spray is a pneumatic spray and the refractory is contained in a slurry having a water base together with a thickening agent. A preferred thickening agent is Cellofas 8.50 which is produced by Imperial Chemical Industries Ltd. Cellofas 8.50 is sodium carboxymethyl cellulose and is available in both granulated and powder form. A possible refractory material which may be used comprises aluminium powder, titanium dioxide, silicon metal powder, silicon carbide boric acid ferrochrome, bichromium trioxide and zirconium silicate.

From another aspect the invention com prises apparatus for applying a protective coating to the surface of a carbon article, the apparatus consisting of means to support and rotate the carbon article and two spray heads, one for aluminium and the other for refractory material, the spray heads being mounted on a carriage, movable longitudinally of the carbon article, the spacing of the spray heads being such that they allow two sprays to reach the surface of the carbon article without interference with each other and also such that the heat of the molten aluminium will not dry out the refractory spray before it reaches the carbon article.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a diagrammatic plan of an apparatus, embodying the invention, for creating a protective coating on the surface of a large carbon electrode.

FIG. 2 is an end elevation, partly in section and on a reduced scale, of the same apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A carbon electrode, of the kind used in large electric furnaces for steel refining, is preheated to about 100 150C to remove moisture and to ensure that when material is subsequently sprayed on to the electrode it dries rapidly.

The three stages of coating then commence. In the first stage there are three steps:

a. Spray aluminium on to the electrode surface.

b. Spray refractory materials on to the aluminum coated surface.

c. Use an electric arc to fuse the refractory and the aluminium.

The second stage repeats the first three steps identically. In the third stage there is no refractory spray but graphite is sprayed. Thus the third stage consists of:

a. Spray aluminium b. Spray graphite c. Fuse the graphite and aluminium by an electric arc.

As shown in the drawings, during the spraying opera tions the electrode 10 which is to be sprayed is supported in a lathe. To support the electrode 10 a nipple 11 is screwed into the central bore at each end of the I electrode 10. This central bore is already threaded because it is normally used to attach one electrode to the next as they are consumed. The electrode surface is prepared by removal of dirt, paint and grease by a very light machining process. During this process gramophone type grooves 12 are formed which assist in the bonding of the material subsequently and also increase surface area.

The carbon electrode surface is then sprayed with aluminium by an electric arc process in which aluminium rods 13, wound on drums 13a, are fed into a gap 14 between arc electrodes 15 and the atomised aluminium is blown by an air jet on to the carbon electrode surface. During spraying the carbon electrode 10 is slowly rotated and a band of aluminium about 1 inch wide is sprayed so as to form continuous spiral coating of aluminium on the electrode surface. A refractory spray head is mounted alongside the aluminium spray, about 4-5 inches laterally displaced from the aluminium spray. A 1 inch band of refractory is sprayed on top of the aluminium at the same time as the aluminium is being sprayed further along the electrode. This gap between the two sprays is necessary for two reasons. Firstly, it avoids the possibility of substantial interference between the aluminium and refractory sprays, and secondly it avoids the drying effect of the heat from the aluminium spray on the refractory spray. If the two sprays were any closer the refractory spray would dry out before it hit the aluminium surface.

The distances separating the two sprays are important, but these are variable and are dependent upon the rate at which the aluminium spray operates, eg at lower current settings, say 200 amperes, the aluminium spraying generator requires a lower volume of air to deposit the aluminium on the electrode, causing the overspray to be much less and consequently, in this case,

the refractory spray gun could be located much closer, approx. 3 inches distant. Conversely, when spraying at 450-500 amps, the separation needs to be 5 inches minimum. ln all cases the spray heads are separated in the vertical plane by some 4 inches.

The aluminium spray and refractory spray heads are both mounted on a carriage which is supported on a grooved rail 21 and by a wheel 22 running in another rail 23. The carriage is traversed automatically by lead screw 24.

The refractory spray is provided by a normal pneumatic paint spry gun, comprising spray head 17 and pipes 18 for the supply of air and 19 for supply of refractory, the refractory being in the form of a slurry contained in a normal paint pressure pot. The slurry may, for example, consist of aluminium powder, silicon carbide, silicon, titanium dioxide, boric acid, ferrochrome, birchromium trioxide, zirconium silicate water and Cellofas. The Cellofas is a thickening agent and is mixed with the water first. The other ingredients are powdered and mixed together and then thoroughly mixed in the Cellofas/water.

This spray technique replaces a technique in which refractory material was hand-painted on to the aluminium. Hand-painting takes much longer and it was necessary to wait until the electrode had been completely coated in aluminium before hand-painting could commence and it was also necessary to wait until the electrode had cooled. The refractory spray can be used simultaneously with the aluminium spray thus cutting out one of the steps in the manufacturing process. The saving in time is particularly valuable after the second aluminium coating because at this stage the time required for cooling prior to hand-painting is quite large.

In the third stage of the process the graphite is applied preferably by spraying.

It is very important in preparing materials for spraying to make sure that the refractory materials are evenly distributed in the slurry. The titanium tends to form small globules which clog the spray nozzle and this can be overcome by adequate mixing of the, titanium with the other powdered materials before they are mixed into the Cellofas/water mixture. The Cellofas/water mixture is made up by mixing 2-2 /2% by weight of Cellofas in warm water at a temperature of about 6080C which ensures that the Cellofas dissolves quickly. Some of the materials used are as follows:

Ferrochrome Boric Acid Bichromium Tioxidc Zirconium Silicate These components are thoroughly mixed in dry powder form. 1250 grams of the dry mixture are then mixed into 1250 c.c. of water which already contains 2-2%% by weight of Cellofas. This quantity of slurry is sufficient to coat an electrode 8 feet long and 24 inches diameter, i.e. about 50 sq. ft. of surface. The above mixture is used for the refractory spray coating.

We claim:

1. A method of creating a protective coating on the surface of a carbon article comprising, in a single operation, directing a spray of aluminium in molten state against said surface, and immediately thereafter directing a spray ofa water-slurry of refractory material onto the aluminium coating so formed without drying out said slurry before it reaches the surface of said carbon article, there being substantially no interference between the sprays.

2. A method according to claim 1 in which the carbon article is an electrode suitable for use in producing steel in an electric furnace.

3. A method as set forth in claim 1 including moving the sprays of aluminium and refractory slurry longitudinally of said carbon article while said carbon article is rotated.

4. A method according to claim 1 in which the aluminium and refractory slurry are sprayed from nozzles which are located alongside each other and are moved relatively to the carbon electrode simultaneously.

5. A method according to claim 4 and in which the distance apart of the spray heads is sufficient to ensure that there is substantially no interference between the sprays of aluminium and refractory material, and also the distance is sufficient to ensure that the heat of the molten aluminium does not dry out the refractory material before it reaches the surface of the carbon article.

6. A method according to claim 1 in which the refractory slurry contains a thickening agent.

7. A method according to claim 6 and in which the thickening agent is sodium carboxymethyl cellulose.

8. A method according to claim 1 in which the refractory material comprises at least one member selected from the group consisting of aluminium powder, titanium dioxide, silicon metal powder, silicon carbide and boric acid, ferrochrome, bichromium trioxide and zirconium silicate.

9. A method according to claim 8 comprising applying a first coating of aluminium and refractory material, the refractory material being in accordance with claim 8, and then applying a second coating of aluminium and refractory material. 

1. A METHOD OF CREATING A PROTECTIVE COATING ON THE SURFACE OF A CARBON ARTICLE COMPRISING, IN A SINGLE OPERATION, DIRECTING A SPRAY OF ALUMINIUM IN MOLTEN STATE AGAINST SAID SURFACE, AND IMMEDIATELY THEREAFTER DIRECTING A SPRAY OF A WATER-SLURRY OF REFRACTORY MATERIAL ONTO THE ALUMINIUM COATING SO FORMED WITHOUT DRYING OUT SAID SLURRY BEFORE IT REACHES THE SURFACE OF SAID CARBON ARTICLE, THERE BEING SUBSTANTIALLY NO INTERFERENCE BETWEEN THE SPRAYS.
 2. A method according to claim 1 in which the carbon article is an electrode suitable for use in producing steel in an electric furnace.
 3. A method as set forth in claim 1 including moving the sprays of aluminium and refractory slurry longitudinally of said carbon article while said carbon article is rotateD.
 4. A method according to claim 1 in which the aluminium and refractory slurry are sprayed from nozzles which are located alongside each other and are moved relatively to the carbon electrode simultaneously.
 5. A method according to claim 4 and in which the distance apart of the spray heads is sufficient to ensure that there is substantially no interference between the sprays of aluminium and refractory material, and also the distance is sufficient to ensure that the heat of the molten aluminium does not dry out the refractory material before it reaches the surface of the carbon article.
 6. A method according to claim 1 in which the refractory slurry contains a thickening agent.
 7. A method according to claim 6 and in which the thickening agent is sodium carboxymethyl cellulose.
 8. A method according to claim 1 in which the refractory material comprises at least one member selected from the group consisting of aluminium powder, titanium dioxide, silicon metal powder, silicon carbide and boric acid, ferrochrome, bichromium trioxide and zirconium silicate.
 9. A method according to claim 8 comprising applying a first coating of aluminium and refractory material, the refractory material being in accordance with claim 8, and then applying a second coating of aluminium and refractory material. 